Power MOS device

ABSTRACT

A semiconductor device comprises a drain, a body disposed over the drain, having a body top surface, a source embedded in the body, extending downward from the body top surface into the body, a gate trench extending through the source and the body into the drain, a gate disposed in the gate trench, a source body contact trench having a trench wall and an anti-punch through implant that is disposed along the trench wall.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/056,346 , entitled POWER MOS DEVICE filed Feb. 11, 2005 now U.S. Pat.No. 7,285,822 which is incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

Power MOS devices are commonly used in electronic circuits. Depending onthe application, different device characteristics may be desirable. Onecommon application is a DC-DC converter, which includes a power MOSdevice as a synchronous rectifier (also referred to as the low side FET)and another power MOS device as a control switch (also referred to asthe high side FET). The low side FET typically requires a smallon-resistance to achieve good power switch efficiency. The high side FETtypically requires a small gate capacitance for fast switching and goodperformance.

The value of a transistor's on-resistance (R_(dson)) is typicallyproportional to the channel length (L) and inversely proportional to thenumber of active cells per unit area (W). To reduce the value ofR_(dson), the channel length can be reduced by using shallower sourceand body, and the number of cells per unit area can be increased byreducing the cell size. However, the channel length L is typicallylimited because of the punch-through phenomenon. The number of cells perunit area is limited by manufacturing technology and by the need to makea good contact to both the source and body regions of the cell. As thechannel length and the cell density increase, the gate capacitanceincreases. Lower device capacitance is preferred for reduced switchinglosses. In some applications such as synchronous rectification, thestored charge and forward drop of the body diode also result inefficiency loss. These factors together tend to limit the performance ofDMOS power devices.

It would be desirable if the on-resistance and the gate capacitance ofDMOS power devices could be reduced from the levels currentlyachievable, so that the reliability and power consumption of the powerswitch could be improved. It would also be useful to develop a practicalprocess that could reliably manufacture the improved DMOS power devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is a cross sectional view of a double-diffused metal oxidesemiconductor (DMOS) device embodiment.

FIG. 2 is a diagram illustrating a buck converter circuit example.

FIGS. 3A-3P are device cross-sectional views illustrating an examplefabrication process used for fabricating device 100 of FIG. 1.

FIG. 4 is a cross sectional view of another DMOS device embodiment inwhich the anti-punch through implant is continuous along the trench walland the trench bottom.

FIG. 5 is a diagram illustrating another DMOS device embodiment thatincludes a Schottky diode in the contact trench.

FIG. 6 is a diagram illustrating another DMOS device embodiment thatincludes a Schottky diode.

FIG. 7 is a device cross sectional view illustrating a device formedusing a double contact etch process.

FIG. 8 is a cross sectional view of another DMOS device embodiment.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess, an apparatus, a system, a composition of matter. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. A component suchas a processor or a memory described as being configured to perform atask includes both a general component that is temporarily configured toperform the task at a given time or a specific component that ismanufactured to perform the task. In general, the order of the steps ofdisclosed processes may be altered within the scope of the invention.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

An improved DMOS device and an associated fabrication process aredisclosed. The device includes a drain, a body and a source. The gate ofthe device is disposed in a gate trench that extends through the sourceand the body into the drain. In proximity of the gate trench andadjacent to the source, there is a source body contact trench with ananti-punch through implant disposed along the trench wall. The topsurface of the gate extends substantially above the top surface of thebody, thus insuring gate-source overlap and allowing source region to beshallow. The process for fabricating the device includes forming a hardmask on a substrate, forming a gate trench in the substrate through thehard mask, depositing gate material in the gate trench, removing thehard mask to leave a gate trench, forming a source body contact trenchhaving a trench wall, and forming an anti-punch through implant.

For the purpose of example, N-channel devices with source and drain madeof N-type material and body made of P-type material are discussed indetail throughout this specification. The techniques and structuresdisclosed herein are also applicable to P-channel devices. FIG. 1 is across sectional view of a double-diffused metal oxide semiconductor(DMOS) device embodiment. In this example, device 100 includes a drainthat is formed on a N⁺-type semiconductor substrate 103, extending intoan epitaxial (epi) layer 104 of N⁻-type semiconductor that is formed onsubstrate 103. Gate trenches such as 111, 113, and 115 are etched in epilayer 104, and gate oxide layers such as 121, 123 and 125 are formedinside the gate trenches. Gates 131, 133 and 135 are disposed insidegate trenches 111, 113 and 115, respectively, and are insulated from theepi layer by the oxide layers. The gates are made of a conductivematerial such as polycrystalline silicon (poly) and the oxide layers aremade of an insulating material such as thermal oxide.

Source regions 151, 153 and 155 are embedded in body regions 141, 143and 145, respectively. The source regions extend downward from the topsurface of the body into the body itself. In the embodiment shown, gate131 has a gate top surface that extends substantially above the topsurface of the body where the source is embedded. Such a configurationguarantees the overlap of the gate and the source, allowing the sourceregion to be shallower than a source region in a device with a recessedgate, and increases device efficiency and performance. The amount bywhich the gate poly top surface extends above the source-body junctionmay vary for different embodiments. The structure is also applicable todevices with gates that do not extend above the top surface of the body.

A set of source body contact trenches 112, 114 and 116 are formedbetween the gates. For example, contact trench 112 penetrates throughsource region 151 forming regions 151 a and 151 b adjacent to the gateand through body region 141 forming regions 141 a and 141 b adjacent tothe trench. During operation, the drain and body regions together act asa diode, referred to as the body diode. A dielectric material layer isdisposed over the gate to insulate the gate from source-body contact.Appropriate dielectric material includes thermal oxide, low temperatureoxide (LTO), boro-phosphosilicate glass (BPSG), etc. The dielectricmaterial forms insulating regions such as 132, 134 and 136 on top of thegates as well as on top of the body and source regions.

In the example shown, the FET channel is formed along the gate trenchsidewall between the source and body junctions. In a device with a shortchannel region, as the voltage between the source and the drainincreases, the depletion region expands and may eventually reach thesource junction. This phenomenon, referred to as punch through, limitsthe extent to which the channel may be shortened. To prevent punchthrough, regions such as 161 a, 161 b, 163 a, 163 b, 165 a and 165 balong the walls of the source body contact trench are heavily doped withP type material to form P⁺-type regions. The P⁺-type regions prevent thedepletion region from encroaching upon the source region. Thus, theseimplants are sometimes referred to as anti-punch through implants. Insome embodiments, to achieve pronounced anti-punch through effects, theP⁺ regions are disposed as close as possible to the channel regionand/or as close as it is allowed by manufacturing alignment capabilityand P⁺ sidewall dopant penetration control. In some embodiments, themisalignment between the trench contact and the gate trench is minimizedby self-aligning the contact, and the trench contact is placed asclosely centered between gate trenches as possible. With thesestructural enhancements, it is possible to shorten the channel such thatthe net charge in the channel per unit area is well below the minimumcharge needed to prevent punch through in an ideal unprotectedstructure. The anti-punch through implants makes it possible toconstruct very shallow trench short-channel devices, thus improvingon-resistance R_(dson) and reducing the gate capacitance. The anti-punchthrough implants also improve body contact resistance.

A layer of metal suitable for making Schottky contact with the lightlydoped drain (such as titanium (Ti), platinum (Pt), palladium (Pd),tungsten (W) or any other appropriate material) is deposited on thebottom of source body contact trenches 112, 114 and 116, to form contactelectrodes 122, 124 and 126, respectively. Since the punch-throughimplants are disposed along the walls of the trenches but not along thebottoms of the trenches, the contact electrodes are in contact with N⁻drain region 104. Together, the contact electrodes and the drain regionform Schottky diodes that are in parallel with the body diode. TheSchottky diodes reduce the body diode forward drop and minimize thestored charge, making the MOSFET more efficient. A layer of metal 180 isdeposited over the Schottky metal to form source body contact. In someembodiments, metal layer 180 is made of aluminum (Al) or made of aTi/TiN/Al stack.

In some embodiments, a single metal that is capable of simultaneouslyforming a Schottky contact to the N⁻ drain and forming good ohmiccontact to the P⁺ body and N⁺ source (e.g. platinum) is used. Thus, theSchottky metal is not necessarily placed in the form of a plug on thebottom of the source-body contact trench. On the other hand, placing thebottom Schottky metal in the form of a plug on the bottom of thesource-body trench can be useful for blocking the anti-punch throughimplant from getting into the N⁻ drain region.

FIG. 2 is a diagram illustrating a buck converter circuit example. Inthis example, circuit 200 is shown to employ a high side FET device 201and a low side FET device 207. High side device 201 includes atransistor 202 and a body diode 204. Low side device 207 is withstructures similar to the one shown in FIG. 1, includes a transistor208, a body diode 210 and a Schottky diode 212. The load includes aninductor 214, a capacitor 216 and a resistor 218. During normaloperation, device 201 is turned on to transfer power from the inputsource to the load. This causes the current to ramp up in the inductor.When device 201 is turned off, the inductor current still flows andcommutates to body diode 210 of device 207. After a short delay, thecontrol circuit turns on device 207, which turns on the channel oftransistor 208 and dramatically reduces the forward drop across thedrain-source terminals of device 208. Without Schottky diode 212, thebody diode conduction loss and the losses from removing the storedcharge in body diode 210 of device 207 can be substantial. However, ifSchottky diode 212 is built into device 207 and if the Schottky diodehas a low forward drop, the conduction loss is greatly reduced. Sincethe low forward drop across the Schottky diode is lower than thejunction drop of the body diode, no stored charge is injected while theSchottky diode conducts, further improving the losses related to dioderecovery.

FIGS. 3A-3P are device cross-sectional views illustrating an examplefabrication process used for fabricating device 100 of FIG. 1. In thisexample, an N type substrate (i.e., an N⁺ silicon wafer with an N⁻ epilayer grown on it) is used as the drain of the device. In FIG. 3A, aSiO₂ layer 402 is formed on N type substrate 400 by deposition orthermal oxidation. The thickness of the silicon oxide ranges from 500 Åto 30000 Å in some embodiments. Other thicknesses are used in otherembodiments. The thickness is adjusted depending on the desired heightof the gate. A photoresist layer 404 is spun on top of the oxide layerand patterned using a trench mask.

In FIG. 3B, the SiO₂ in the exposed areas is removed, leaving a SiO₂hard mask 410 for silicon etching. In FIG. 3C, the silicon is etchedanisotropically, leaving trenches such as 420. The gate material isdeposited in the trenches. The gate that is later formed within thetrench has sides that are substantially perpendicular to the top surfaceof the substrate. In FIG. 3D, SiO₂ hard mask 410 is etched back by anappropriate amount so that the trench walls remain approximately alignedwith the edge of the hard mask after later etching steps. SiO₂ is themask material used in this embodiment because etching using a SiO₂ hardmask leaves relatively straight trench walls that mutually align withthe sides of the mask. Other material may be used as appropriate.Certain other types of material traditionally used for hard masketching, such as Si₃N₄, may leave the etched trench walls with acurvature that is less desirable for gate formation in the followingsteps.

In FIG. 3E, the substrate is etched isotropically to round out thebottoms of the trenches. The trench is approximately between 0.5-2.5 μmdeep and approximately between 0.2-1.5 μm wide in some embodiments;other dimensions can also be used. To provide a smooth surface forgrowing gate dielectric material, a sacrificial layer of SiO₂ 430 isgrown in the trenches. This layer is then removed by the process of wetetching. In FIG. 3G, a layer of SiO₂ 432 is grown thermally in thetrenches as dielectric material.

In FIG. 3H, poly 440 is deposited to fill up the trenches. In this case,the poly is doped to obtain the appropriate gate resistance. In someembodiments, doping takes place as the poly layer is deposited (insitu). In some embodiments, the poly is doped after the deposition. InFIG. 3I, the poly layer on top of the SiO₂ is etched back to form gatessuch as 442. At this point, top surface 444 of the gate is stillrecessed relative to top surface 448 of the SiO₂; however, top surface444 of the gate is higher than top layer 446 of the silicon. In someembodiments, no mask is used in poly etch back. In some embodiments, amask is used in poly etch back to eliminate the use of an additionalmask in the following body implanting process. In FIG. 3J, the SiO₂ hardmask is removed. In some embodiments, dry etch is used for hard maskremoval. The etching process stops when the top silicon surface isencountered, leaving the poly gate extending beyond the substratesurface where source and body dopants will be implanted. In someembodiments, the gate extends beyond the substrate surface byapproximately between 300 Å to 20000 Å. Other values can also be used. ASiO₂ hard mask is used in these embodiments since it provides thedesired amount of gate extension beyond the substrate surface in acontrollable fashion. A screen oxide may then be grown across the wafer.

In FIG. 3K, a photoresist layer 450 is patterned on the body surfaceusing a body mask. The unmasked regions are implanted with body dopant.Dopant material such as boron ions is implanted by bombarding thesubstrate surface with the dopant material, or any other appropriateimplantation methods. The photoresist is then removed and the wafer isheated to thermally diffuse the implanted body dopant via a processsometimes referred to as body drive. Body region 460 is then formed. Insome embodiments, the energy used for implanting the body dopant isapproximately between 30-200 Kev, the dose is approximately between5E12-4E13 ions/cm², and the resulting body depth is approximatelybetween 0.3-2.4 μm. Other depths can be achieved by varying factorsincluding the implant energy and dose. In some embodiments, a mask isnot used in body implantation.

In FIG. 3L, a photoresist layer 610 is patterned to allow source dopantto be implanted in region 612. In this example, arsenic ions penetratethe silicon in the unmasked areas to form N⁺ type source. In someembodiments, the energy used for implanting the source dopant isapproximately between 5-80 Kev, the dose is approximately between1E15-1E16 ions/cm², and the resulting source depth is approximatelybetween 0.05-0.5 μm. Further depth reduction can be achieved by varyingfactors such as the doping energy and dose. The photoresist is thenremoved and the wafer is heated to thermally diffuse the implantedsource dopant via a source drive process. Other implant processes mayalso be used as appropriate. In FIG. 3M, a dielectric (e.g. BPSG) layer620 is disposed on the top surface of the device after source drive, anddensified if needed. An etch mask 614 is then formed.

In FIG. 3N, contact trench etch is performed to form trenches such as622, 624 and 626. Sections of the source implant and the body implantare etched away in the appropriate areas. In FIG. 3O, punch-throughprevention implants 630 and 632 are formed along the vertical walls ofcontact trenches 622 and 624. In some embodiments, the implants aredeposited by bombarding ions at an angle onto the sidewalls of thetrenches. In other embodiments, implants 630 and 632 are formed using acontact etch process, which is described in more details below. In FIG.3P, a metal stack such as Ti+TiN+Al—Si—Cu is disposed to form a contact.A mask etch 640 separates the gate metal contact from the source-bodycontact. Since the trench such as 624 serve as contact openings wherethe metal and the semiconductor meet, sharp curvature in corner regionsmay lead to high electric fields and degrade device breakdown. In device350 shown, the trenches have round and smooth shapes without sharpcomers, thus avoiding low breakdowns due to high electric fields.

FIG. 4 is a cross sectional view of another DMOS device embodiment inwhich the anti-punch through implant is continuous along the trench walland the trench bottom. In this example, a layer of P⁺ material 402 isformed along the source-body contact trenches of device 490. In someembodiments, the P⁺ layer is formed by bombarding the trench surfacewith P⁺-type material. In some embodiments, the trench and the P⁺ layerare formed by disposing P⁺-type material in the body region before thetrench is formed and then etching away the P⁺-type materialappropriately. A layer of contact metal 404 (such as Ti or TiN) isdisposed in the trenches as well as on top of the gate oxide. Thetrenches are filled with material such as W. A layer of contact metal(such as Al—Si—Cu) is disposed. The depth of the trench may vary and canexceed the depth of the gate in some embodiments. Deeper trench canprovide better shielding of the channel area. Although no Schottky diodeis formed in this device, the device has low R_(dson) and is used as ahigh side FET in some circuits.

FIG. 5 is a diagram illustrating another DMOS device embodiment thatincludes a Schottky diode in the contact trench. In device 500 shown inthis example, P⁺-type material is disposed at an angle such that theanti-punch through implants 502 and 504 are formed along the trenchwalls and not in the trench bottom. Contact metal layer 506 and drain508 form a Schottky diode with a low forward drop voltage.

FIG. 6 is a diagram illustrating another DMOS device embodiment thatincludes a Schottky diode. In this example, plugs 602 and 604, which maybe made of poly, oxide or like material, are disposed in the contacttrench of device 600. Implants 606 and 608 are formed along the trenchwall by bombarding the trench wall with P⁺-type material. Plugs 602 and604 prevent the bombarded P⁺ ions from extending much below the topsurfaces of the plugs, allowing the implants to form along the trenchwalls but not in the trench bottom. Schottky diodes are formed bycontact electrode 610 and drain 612.

FIG. 7 is a device cross sectional view illustrating a device formedusing a double contact etch process. In this example, contact trenchetch process is performed on a structure similar to 340 of FIG. 3M toform device 700. After etch mask 614 is formed on the structure, contacttrench etch is performed to form trench 625. The depth of the trench mayvary for different implementations. In the example shown, the bottom oftrench 625 is controlled to be substantially coplanar to the sourcebottom. P⁺-type material is implanted to the bottom of the trench andthen activated to form P⁺ region 607. A second contact trench etch isperformed to etch the trench through the body region to the N⁻ drain.Metal layers are then deposited to form structures such as 350 of FIG.3P, 500 of FIG. 5 or 600 of FIG. 6. A Shottky diode is formed betweenthe trench metal and N⁻ drain.

FIG. 8 is a cross sectional view of another DMOS device embodiment. Inthe example shown, the double contact etch technique is used to etch thetrench substantially through the P⁺ implant region 607. The residual P⁺implant region forms ohmic contact with metal layers deposited insidethe trench. Similar to the device 490 of FIG. 4, DMOS device 800 doesnot include an integrated Schottky diode. The residual P⁺ regionprovides good punch through shield. Since there is no P⁺ region on thebottom, the device has lower injection efficiency therefore the bodydiode stored charge is greatly reduced.

A DMOS device and its fabrication have been disclosed. The techniquesare also applicable to other semiconductor device types such asInsulated Gate Bipolar Transistors (IGBTs) and MOS-Controlled Thyristors(MCTs) where shielding the channel area using a punch through preventionimplant is desirable.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

1. A semiconductor device comprising: a drain; a body disposed over thedrain, having a body top surface; a source embedded in the body,extending downward from the body top surface into the body; a gatetrench extending through the source and the body into the drain; a gatedisposed in the gate trench; and a source body contact trench having atrench wall, a trench bottom, and an anti-punch through implant that isdisposed along the trench wall; wherein: the source body contact trenchincludes an electrode that is disposed within the source body contacttrench; the electrode is at least in part in physical contact with thesource and at least in part in physical contact with the anti-punchthrough implant; the anti-punch through implant is disposed along atleast a section of the trench wall but not along at least a portion ofthe trench bottom.
 2. The semiconductor device of claim 1, wherein thesource body contact trench is in proximity of the gate trench andadjacent to the source.
 3. The semiconductor device of claim 1, whereinthe gate has a gate top surface that extends substantially above thebody top surface.
 4. The semiconductor device of claim 1, wherein thesource has a source top surface, and the gate has a gate top surfacethat extends substantially above the source top surface.
 5. Thesemiconductor device of claim 1, wherein the electrode includes a metalthat is suitable for providing Ohmic contact to the source and the body.6. The semiconductor device of claim 1, wherein the electrode includes ametal that is suitable for forming a Schottky diode with the drain. 7.The semiconductor device of claim 1, wherein the electrode includes amaterial that is suitable for forming a Schottky diode with the drain,and the material forms a plug in the bottom of the source body contacttrench.
 8. The semiconductor device of claim 1, wherein the electrodeincludes a metal that is suitable for both providing Ohmic contact tothe source and the body and for forming a Schottky diode to the drain.9. The semiconductor device of claim 1, wherein the electrode includesplatinum.
 10. The semiconductor device of claim 1, wherein the electrodeand the drain form a Schottky diode.
 11. The semiconductor device ofclaim 1, wherein the electrode and the drain form a Schottky diodesituated below a body diode of the device.
 12. The semiconductor deviceof claim 1, wherein the electrode and the drain form a Schottky diode,and wherein a metal layer is disposed over the electrode to form asource body contact.
 13. The semiconductor device of claim 1, whereinthe source body contact trench extends through the body to the drain.14. The semiconductor device of claim 1, wherein the source body contacttrench is formed to have a smooth shape.
 15. The semiconductor device ofclaim 1, wherein the anti-punch through implant includes a regionheavily doped with P type material.
 16. The semiconductor device ofclaim 1, wherein: the anti-punch through implant has a cross sectionaldepth measured in the direction perpendicular to the trench wall of thesource body contact trench, and a cross sectional length that ismeasured in the direction along the source body contact trench wall; andthe cross sectional depth is substantially less than the cross sectionallength.